Thermoplastic polyesters, such as PBT (polybutylene terephthalate) and PET (polyethylene terephthalate) possess excellent dimensional-stability, heat-resistance and chemical-resistance properties which are used in the electrical, electronic and motor-vehicle fields. However, at high temperature, during conversion operations, a reduction in the molecular weight of the polymer may occur, leading to a reduction in the impact strength properties. In addition, polyesters have poor fracture-resistance properties in the case of notched components.
The present invention provides thermoplastic polymers in which an impact-modifier composition is added in order to obtain improved impact properties, including low-temperature toughness. The present invention also relates to this impact-modifier composition that is added to the polyesters to improve the impact properties thereof. These modifier compositions make it possible to achieve impact properties superior to those obtained with each of the compounds separately.
Patent U.S. Pat. No. 4,753,890 (=EP 174,343) describes polyesters, such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT) modified by ethylene-alkyl (meth)acrylate-glycidyl (meth)acrylate copolymers.
Patent EP 737,715 describes PBTs modified by a blend of an ethylene-methyl methacrylate-glycidyl methacrylate copolymer and of a copolymer of the core-shell type. These core-shell copolymers comprise fine particles having an elastomer core and a thermoplastic shell.
Patent EP 531,008 describes PBT/polycarbonate blends containing copolymer core-shells and copolymers which are either ethylene-glycidyl methacrylate copolymers or ethylene-vinyl acetate-glycidyl methacrylate copolymers.
Patent U.S. Pat. No. 5,369,154 describes PET/polycarbonate blends containing four different modifiers: a copolymer comprising an epoxide, a copolymer core-shell, an SBR- or SBS- or EPR-type elastomer and an SAN- or ABS-type copolymer.
Patent EP 115,015 describes PET or PBT containing linear low-density polyethylene (LLDPE), glass fibres and optionally a core-shell copolymer.
Patent EP 133,993 describes PET containing a core-shell copolymer and a copolymer of ethylene with either an alkyl acrylate or (meth)acrylic acid.
Japanese Patent Application JP 01,247,454 A, published on 3 Oct. 1989 describes PBT containing an ethylene-alkyl (meth)acrylate copolymer and an ethylene-glycidyl methacrylate copolymer.
Patents EP 838,501 and EP 511,475 describe compositions similar to those of the above Japanese application.
Patent EP 803,537 describes PET and polycarbonate containing a copolymer comprising glycidyl methacrylate. Firstly, the polycarbonate and the copolymer comprising glycidyl methacrylate are blended together and then this blend is incorporated into the PET.
Patent EP 187,650 describes PET containing a core-shell copolymer and a copolymer of ethylene with either maleic anhydride or a (meth)acrylic acid.
It has been seen from the prior art that saturated polyesters can have their impact properties improved by the addition of a core-shell copolymer. These polymers have a particularly well defined structure in which the core consists of a polymer having an elastomeric character and in which the shell has a thermoplastic character. It has also been seen that the improvement in impact strength may be obtained by also incorporating a dispersed phase of an impact modifier optionally containing reactive functional groups capable of reacting with the functional groups of the polyesters. This reactivity makes it possible to ensure a fine and homogeneous dispersion of the modifier as well as good adhesion. The core-shell copolymer may itself also be functionalized in order to allow better adhesion to the matrix. However, this reactivity is sometimes high and may lead to a reduction in the melt flow index. This reduction in the melt flow index is prejudicial to the injection moulding of large parts or of fine parts.
It has now been found that it is possible to improve the impact properties of thermoplastic polyesters by adding to them three kinds of modifier, namely: (a) a core-shell copolymer, (b) an ethylene-unsaturated epoxide copolymer or an ethylene-unsaturated carboxylic acid anhydride copolymer or blends thereof and (c) an ethylene-alkyl (meth)acrylate copolymer or an optionally neutralized ethylene-(meth)acrylic acid copolymer or blends thereof. This modification does not result in a drop in the melt flow index compared with the prior art and even improves it. These modifiers improve the impact strength properties either at room temperature or at low temperatures, depending on the ratio which is chosen between the three components (a), (b) and (c), compared with compositions encountered in patents EP 511,475 and EP 174,343. They also allow the material to have better melt flow compared with compositions as described in EP 737,715.
The present invention relates to thermoplastic polyester compositions comprising, by weight, the total being 100%:                60 to 99% of a thermoplastic polyester;        1 to 40% of an impact modifier comprising:                    (a) a core-shell copolymer (A);            (b) an ethylene copolymer (B) chosen from ethylene-unsaturated carboxylic acid anhydride copolymers (B1), ethylene-unsaturated epoxide copolymers (B2) and blends thereof;            (c) a copolymer (C) chosen from ethylene-alkyl (meth)acrylate copolymers (C1), optionally neutralized ethylene-(meth)acrylic acid copolymers (C2) and blends thereof.                        
The present invention also relates to an impact-modifier composition which can be added to thermoplastic polyesters to improve their impact properties and which comprise:                (a) a core-shell copolymer (A);        (b) an ethylene copolymer (B) chosen from ethylene-unsaturated carboxylic acid anhydride copolymers (B1), ethylene-unsaturated epoxide copolymers (B2) and blends thereof;        (c) a copolymer (C) chosen from ethylene-alkyl (meth)acrylate copolymers (C1), optionally neutralized ethylene-(meth)acrylic acid copolymers (C2) and blends thereof.        